This invention pertains generally to the field of cutting tools and, more specifically, it pertains to cutting tools known as reamers and associated reaming methods.
Reamers are cutting tools that are typically used to perform the final cutting operation on holes, particularly holes with small tolerances. Specifically, reamers perform secondary cutting operations after a hole has been drilled close to a desired final size. Reamers therefore typically have an external diameter that is slightly larger than that of the drilled hole, and are designed to finish the hole to within a small tolerance of the desired size and to provide a relatively smooth inner wall.
Standard reamers typically include a shank, a body portion at one end of the shank, and a plurality of teeth, such as 4-8 teeth, that are spaced around the body portion and extend outwardly therefrom to form the cutting surfaces of the reamer. Each tooth includes a rake face and a cutting edge that actually engages the workpiece in the course of reaming a hole. In some cases, the teeth are uniformly spaced around the body portion. In other cases, the teeth have non-uniform or irregular spacing. However, reamers of each type suffer from a number of problems in precision applications, such as reaming holes with small tolerances. These problems become particularly troublesome in applications that require a number of precisely toleranced holes to be formed, such as during the construction of airframe components.
One problem with standard reamers having evenly spaced teeth is that such reamers typically produce finished holes that have a lobed pattern. Lobing is caused in part by vibration and unequal rubbing forces on the reamer during operation, and often create form error that exceeds the allowable size tolerance specified in a given application, such as the tolerances specified in ASME Y14.5. Specifically, the form error occurs when the reamer axis orbits about the axis of the hole in a whirling motion at integer multiples of the number of teeth (N) per revolution, resulting in a hole form with N+1 or Nxe2x88x921 lobes depending on whether the reamer axis orbits in the hole in a backward or forward direction, respectively. Typically, the reamer is rotated about its longitudinal axis in a forward, or clockwise, direction, while the reamer orbits about the hole axis in a backward, or counterclockwise, direction. In addition, the lobes continue to regenerate as the reamer progresses through the hole. See FIG. 7a. 
Irregular pitch reamers address the lobing problems caused by reamers with uniform teeth spacing. Specifically, irregular pitch reamers use non-uniform teeth spacing to reduce the whirling of the reamer, thereby improving roundness control. As a result of the irregular teeth spacing, the reamer axis oscillates about the hole axis once per tool revolution compared to the multiple oscillations of a standard reamer having evenly spaced teeth as described above. However, while the roundness of the hole is improved, the irregular spacing causes an imbalance in the forces exerted on the reamer. This force imbalance causes the reamer axis to be offset from the hole axis, thereby producing oversized holes that can also exceed the specified tolerance of a given application. In particular, each tooth has a force vector with magnitude and direction components as shown in FIG. 4. When summed, the force vectors of an irregular pitch reamer are imbalanced, thereby creating a net tool offset force that causes the reamer to be offset from the hole axis. This offset, in turn, causes the resulting hole to be oversized.
Thus, conventional reamers do not produce holes that consistently have the roundness and the precise size that are desired. Instead, standard reamers with regularly spaced teeth generally create lobes in the finished surface of the hole, while irregular pitch reamers typically create round finished holes with an oversize condition. It would therefore be desirable to provide a reamer that creates a smooth finished surface for a drilled hole that is sufficiently round and is sized within tolerance.
These and other needs are provided, according to the present invention, by a force balanced irregular pitch reamer that substantially reduces or eliminates the formation of lobes in the finished surface of a formed hole without causing an oversize condition. As such, the reamer is particularly well suited for precision cutting operations requiring small tolerances in both size and form.
According to the present invention, a reamer, such as a force balanced irregular pitch reamer, is provided that comprises a shank, a body portion connected to one end of the shank, and a plurality of teeth projecting outwardly from a longitudinally extending portion of the body. The teeth of the reamer are preferably non-uniformly spaced in order to reduce lobing. Each tooth has a rake face and a cutting edge and defines a rake angle between the rake face and an imaginary line extending between the longitudinal axis of the reamer and the cutting edge. According to the present invention, the rake angle for a particular tooth varies from that of another tooth, such as by at least 2xc2x0, such that the cutting force vector established by each tooth are substantially balanced, thereby permitting the longitudinal axis of the reamer and the hole axis to be substantially colinear so that a hole of the desired size can be reamed. For example, the teeth of one embodiment define rake angles of 0xc2x0, 6xc2x0, and 7xc2x0. Typically, however, the teeth do not define rake angles that exceed about 10xc2x0.
In operation, the force balanced reamer is rotated at a low to moderate rate about its longitudinal axis and is inserted into a formed hole sized slightly smaller than the reamer. The formed hole may have been formed by many methods, such as by drilling. The reamer is then advanced through the formed hole to provide a hole having the desired size and a smooth finished surface. The reamer engages the workpiece and each tooth is subjected to cutting forces. Specifically, each tooth encounters a cutting force having a magnitude and direction, thus defining a cutting force vector. The cutting force vector for each tooth is at least partially determined by the rake angle of the tooth. By summing the cutting forces that act on the teeth, the resulting effect upon the reamer can be determined.
Advantageously, the rake angles of the teeth are defined such that the sum of the cutting force vectors are approximately balanced in at least two dimensions. Thus, each tooth is subjected to a cutting force, but the rake angle of each tooth is formed in cooperative arrangement with the rake angles of the other teeth such that the sum of the cutting force vectors for all the teeth tend to negate or cancel one another, thereby balancing the reamer. As such, the reamer of the present invention creates properly sized holes since the substantial balance of the cutting force vectors substantially reduces any offset between the longitudinal axis of the reamer and the hole axis. Since the reamer of the also generally present invention includes irregularly spaced teeth, the holes created by the reamer are also generally free of lobes.